Jeff Aaron explains how WAN Acceleration can assist in developing more effective IT continuity solutions.

As companies become more reliant on business critical information, it becomes increasingly imperative to protect resources against unexpected loss. Threats come in many shapes and sizes – from server maintenance to unexpected network outages to catastrophic natural disasters that can debilitate an entire building or region. Significant emphasis is being placed on business continuity management as a way of addressing these concerns. As part of this process, CIOs are taking a hard look at their IT infrastructure to ensure that data is protected and easily recoverable in the event of an unfortunate event or disaster.

Data backup and replication are two solutions that are essential to most enterprises’ disaster recovery plans. However, given the fact that both of these solutions typically involve the transfer of large amounts of information across significant geographic distances, limitations in WAN technology can make it difficult to implement them effectively. By overcoming these challenges, WAN acceleration facilitates disaster recovery, making it an essential component of most enterprises’ business continuity plans.

Common characteristics
Both network backup and data replication (synchronous and asynchronous) have common characteristics that can impact the effectiveness of these solutions when delivered across a WAN. These include:

* Large volumes of data
When performing backup and replication, database files, control files, and other information must be transferred across the WAN. As a result, WAN links are forced to handle hundreds of Megabytes (or Terabytes) of data when doing disaster recovery. To accommodate this enormous volume of data, enterprises will typically deploy large WAN links between data centres – e.g., 45 Mbps or higher. Given the price of WAN bandwidth, this often becomes the most expensive component of a disaster recovery solution.

In some instances, backups can be postponed until non-peak hours to assist with this problem. In many instances, however, this is not an option, as backups must be performed regularly (e.g., every hour) for compliance reasons. Furthermore, data replication is most effective if performed in real-time, eliminating the ability to postpone this function until off-hours.

* Sensitive to bandwidth and latency
When performing synchronous data replication, the primary server cannot continue to write until the secondary server finishes writing and sends an acknowledgment. As a result, the process is highly subjective to WAN latency. Even asynchronous replication and network backup can suffer from high latency as transfers can time-out across the WAN, leading to file transfer failures and subsequent database synchronisation problems that can be difficult, and time consuming, to troubleshoot.

The Transport Control Protocol (TCP) often requires significant tweaking to run on WAN links with high latency and/or low bandwidth. As a result, many data replication solutions default to the User Datagram Protocol (UDP) instead of TCP.

* Repetitive information
A significant portion of information sent across the WAN for disaster recovery purposes is repetitive. As a result, many solutions transfer only data blocks that have changed since the previous backup/replication. Incremental changes can significantly reduce the amount of traffic traversing the WAN, which speeds up the back/replication process.

It is important to note, however, that backup/replication solutions will examine large blocks of information to determine what is incremental. They do not have the same level of granularity (and therefore WAN efficiency) as other solutions that can detect repetition at the individual byte level. For instance, if a single byte edit is inserted into a file, all of the blocks within the file will change. Block level data reduction will not catch this change, and therefore cannot reduce the amount of data transferred across the WAN in this scenario. Byte level solutions, on the other hand, can detect deltas down to a single byte, detecting the slightest changes for maximum WAN efficiency.

In addition, incremental backups require a full backup as a baseline. If the full backup is compromised or out of date, the incremental backups are useless. As a result, it is essential to perform full backups fairly regularly – once a week for example. In the event of a disaster, it is the full data set that is often required to restore the main servers. As a result, the WAN must be able to handle large volumes of data, above and beyond what is sent as incremental changes.

These characteristics can make it difficult to perform disaster recovery across a WAN. As a result, they can compromise disaster recovery plans by reducing the frequency of backups/replication. Or, in some instances, they increase the cost of performing disaster recovery as IT resources are required to troubleshoot and correct errors, as is the case with database synchronisation problems. Either way, enterprises can be exposed to a vulnerability gap that may compromise their business continuity plans.

* Improve data transfer times
Data reduction technology can be used to recognise repetitive information and deliver it from local data stores (as opposed to re-sending it across the WAN). This enables WAN transfers to be handled at LAN-like speeds. More advanced solutions perform data reduction on both TCP and UDP traffic, accommodating all backup and replication platforms and configurations.

Figure 2. WAN acceleration can be used to recognise duplicate information and deliver it locally, improving the transfer time of data across the WAN. CLICK HERE FOR A LARGER VERSION

In addition to data reduction, enhancements can be implemented that accelerate TCP and CIFS traffic, including selective acknowledgement, read-aheads, and write-behinds. This reduces the impact of latency on data transfers using these protocols.

* Maximise WAN efficiency
Data reduction can reduce as much as 99 percenr of WAN traffic by eliminating the transfer of duplicate information. When performed at the byte level, repetitive patterns can be detected and eliminated even when the backup/replication solution is performing similar functions at the block level.

In addition, advanced compression techniques can further reduce the amount of WAN bandwidth required for backup and replication. These techniques work in both directions of a WAN link, improving the backup/replication process while ensuring that the WAN can efficiently handle a restore if needed. By providing compression within the acceleration appliance, this functionality can be offloaded from the host replication server, ensuring better scalability and performance. In addition, significant performance improvement can be provided even when non-repetitive information is sent across the WAN.

* Reduce packet loss and delivery errors
WAN acceleration reduces the impact of both packet loss and jitter that occurs when router links are oversubscribed and drop or re-order packets. This is often handled via adaptive Forward Error Correction (FEC), a technology that is used to reconstitute lost packets at the far end of a WAN link, avoiding delays that come with multiple-round-trip retransmissions. Some solutions dynamically adjust the FEC overhead in response to changing link conditions for maximum effectiveness in environments with high packet loss.

Conclusion
Data reduction, compression, and latency/loss mitigation are all essential for overcoming WAN performance limitations. This is particularly true when handling large volumes of data, as is the case with disaster recovery.

By leveraging a full suite of WAN acceleration techniques, enterprises can cost effectively support all backup/replication environments – including data centre to data centre transfers and network backup across many remote sites. In addition, by accelerating both TCP and UDP traffic, WAN acceleration can be a perfect complement to any disaster recovery application.

Jeff Aaron is director of product marketing, Silver Peak.

Silver Peak Systems is exhibiting at Storage Expo 2006 the UK's largest event dedicated to data storage. Now in its 6th year, the show features a comprehensive free education programme and over 90 exhibitors at the National Hall, Olympia, London from 18 - 19 October 2006 www.storage-expo.com

•Date: 13th October 2006• Region: World •Type: Article •Topic: IT continuity
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